The invention relates in general to a method and apparatus for detecting very small gas leaks and in particular to a method and apparatus for detecting leakage using nuclear magnetic resonance.
A leak results from a hole or porosity in an enclosure or, more generally, an article of manufacture capable of passing material from the higher pressure side to the lower pressure side. A leak path normally has an involved geometry sometimes extending a relatively long distance from beginning to end. Leakage refers to the flow of a material through a leak path without regard to the physical size or shape of the hole. Leakage typically occurs as a result of a pressure differential across the leak path. However, capillary effects can also be the cause of leakage.
Nothing can ever be completely free of leakage. Every container or article always has some leakage, even if those leaks are so minute that it would take hundreds of years for a cubic centimeter of gas to escape. The high cost of finding small leaks must be balanced against the functioning of the article over its useful life.
Because the cause of a leak usually cannot be seen or physically measured, the quantity often used to describe the leak is the leakage rate. The concept of an object being leakage tight is meaningless except in relation to the substance which is to be contained under operating conditions and the objectives with respect to safety, contamination and reliability. A measure of leakage rate must have dimensions equivalent to pressure, temperature, time, and volume. Leakage tight or acceptable leakage rate is the practical leakage which is acceptable under normal operating circumstances.
Manufacturing of articles which are meant to hold liquids or gases, i.e., serve some sort of barrier function, requires that the article be tested for a leakage in order to practice quality assurance. The demands of such quality assurance testing vary greatly depending on the nature of the article and its intended use.
In a high volume production of articles having a stringent permissible leakage rate the cost associated with high accuracy leak detecting is weighed against the competing cost of time taken to perform the test. That is, it is desired to test each article at the accuracy required by manufacturing specifications but to do so at a minimal cost of materials and time. Typical products for which economic leakage detection is a significant issue are vacuum chambers, TV and other cathode ray tubes, hermetically-sealed electronic components, pressure vessels, aerosol containers, vacuum thermal isolation, semiconductor manufacturing equipment and automotive gas tanks.
There are many different methods of leak testing some of which allow a quantitative determination of leakage and others give a qualitative leakage determination or are otherwise highly dependent on subjective determinations. Qualitative leak tests include bubble testing. A soap solution is painted over the surface of a pressurized vessel or the vessel is immersed in a tank of liquid. In soap bubble testing the formation of bubbles indicates the area of the article which is leaking. The leakage rate, however, is difficult to estimate and the article under test must be cleaned after the test. Hydrostatic testing involves filling the article under test with water at high pressure and looking for moisture formation due to leakage. This test requires a long test cycle since there is a significant cleanup and the test itself is relatively time consuming.
Some of the above tests can be evaluated to give a semi-quantitative result but require the operator to apply subjective judgements to do so.
There are also a number of fully quantitative methods of leak detection in use. It is convenient to rank these according to the minimum detectable leakage rate achieved with each technique. In the pressure-drop technique, the test system pressurizes the article with dry air and uses a suitable pressure sensor to measure the pressure change due to subsequent leakage. A refinement is to simultaneously pressurize two articles, the one to be tested and the other an identical article known to be leak-free. Subsequent pressure difference between the two parts allows a leakage rate to be measured which is substantially independent of temperature changes caused by the pressurization process itself.
The mass-flow technique is generally similar to pressure-drop, but measures differential mass flow between the two articles. It has advantages in higher speed, and in offering an immediate measurement of leakage rate independent of test pressure. The mass flow technique also enables the use of further refinements to minimize errors introduced by heat exchange between the walls of the article and the air within, particularly when the temperature of the walls is changing significantly during the period of test. The effectiveness of such refinements varies, however, with the size of the article, the time available for testing, the test pressure, and the rate of change of temperature in the article. In general, neither of the above techniques is likely to give accurate measurements of leakage rates much smaller than 10xe2x88x925 sccs (standard cc. per second).
For smaller leakage rates to be detectable, a tracer-gas technique must be used. The leak test system is arranged to produce a different concentration of the chosen tracer-gas between the interior and the exterior of the article. The essential difference form the techniques described above, is that pressure (or vacuum) is used merely to produce a pressure-gradient tending to drive molecules through the leak, not as a measurement means. A detector, chosen to be sensitive to the tracer gas only, is used to measure the passage of molecules of the tracer gas through any leak. In a typical system, a tracer gas such as sulfur hexafluoride, hydrogen, or helium is diluted with dry air (for economy) and injected to create an over-pressure inside the article. The system includes means outside the article for collecting any of the gas mixture emerging through a leak, and passing it to the detector. Provided the detector is sufficiently selective, the tracer gas technique is completely independent of changes of temperature and/or pressure in the part or article under test or the collection system.
Various means are used to make a detector sufficiently selective for the chosen tracer. For example, a mass spectrometer can be set to match the charge/mass ratio for the nuclei of hydrogen or helium. This arrangement is highly selective and leakage rates down to 10xe2x88x9211 sccs. are measurable. A significant drawback however is that the tracer molecules must first be ionized, then detected. This normally requires high vacuum conditions, achieving which generally introduces severe time and cost penalties in production-line leak-testing. Care is also required to ensure low background concentrations of tracer in the environment, and rapid clean-up after the testing of a previous article having a gross leak.
Other known operational goals include minimizing the costs of: test setup; consumables used during testing; and cleanup. Where a leak detection system uses tracer gases it is advantageous if the gases are non-toxic, non-flammable and as inexpensive as practical.
An apparatus and method embodying the present invention provide for measuring a leakage rate in which the nuclei of tracer gas are detected by means of their overall spin, i.e. nuclear magnetic moment. This affords even higher selectively than a mass spectrometer, since only a small minority of all possible gases have nuclei with overall spin. There is no need to ionize the gas, nor to develop a high-vacuum since the main requirement for detection by nuclear magnetic resonance is simply to collect a sufficiently high number of polarized nuclei within the detector.
The tracer gas, according to a preferred embodiment of the present invention, is a stable isotope of one of the noble gases such as a helium-3 which is stable, i.e. non-radioactive, and has spin of xc2xd with a nuclear magnetic moment of xe2x88x922.127. Polarized tracer gas can be detected by applying a strong magnetic field and radio-frequency excitation and measuring the resonant frequency of electromagnetic energy emitted during subsequent relaxation. This frequency is dependent on the spin, the moment, and the applied field, but (to first order) nothing else, thus offering exceptional selectivity.
However, at thermal equilibrium such as a gas has few spin-polarized nuclei, making the detection process rather insensitive. Accordingly in one aspect of the present invention helium-3 is hyper-polarized by combining it with a vapor of polarized metal atoms, such as a vapor of rubidium, Rb, which has been optically polarized with a laser array. In optical polarization of the rubidium, Rb, the atoms are exposed to circularly polarized light at a wavelength of approximately 795 nm. The metal atoms transfer the polarization to the noble gas atoms"" nuclei through collisions.
The hyper-polarization of helium gas generates a tracer gas with ten thousand times more atoms which are detectable by the nuclear magnetic resonance detection system of the present invention. This coupled with the lack of naturally occurring helium in the environment makes the detection system of the present invention relatively insensitive to environmental background gases and impurities.
The method and apparatus embodying the present invention combine the usual advantages of using He as a tracer gas with other advantages such as the ease of using an NMR detection system at near-atmospheric pressure. Using helium gas in leak testing enables detection of very low leak rates because the relatively small molecular structure of helium, which allows the gas to pass easily through pores that would block larger molecules of most other air component gases such as oxygen and nitrogen. Additionally, helium is chemically inert so none is present in polymers or plasticizers and no adsorption occurs onto metal or polymers surfaces of the apparatus of the invention.
Another aspect of the present invention is to minimize test cycle time in order to decrease the costs of leak rate testing. Test cycle time is the time to complete a leak detection test of one manufactured article and begin the test of the next article.
In still other aspects of the present invention the leak detection system provides an apparatus and method having reliability of measurement values, sensitivity to small leaks, and insensitivity to temperature variations and environmental gases present either as environmental background or outgassed from the article involved in testing.